Method of bleaching paper pulp



Patented Dec. 6, 1927.

UNITED STATES" 'o'rro mass, or Arrnnron, wrscousnv',

ASSIGNOR TO AMERICAN LAKES PAPER COMPANY, OF CHICAGO, ILLINOIS, A COBI'ORATIOH OF DELAWARE.

No Drawing.

' it is impractical to bring this pulp to a satisfactory white color through the use of bleaching powder or like oxidizing chlorine bleaching, agents without decidedly impair ing its characteristic strength and toughness and without materially reducing the yield.

The present improvement seeks to provide a method of bleaching chemical wood pulp and more particularly sulphate or kraft pul to produce a satisfactory light color witout serious deterioration either in the yield or in the strength of the pulp.

The present inprovement consists in first bleaching the pulpin the ordinary way with a chlorine bleaching agent, preferably bleaching powder, the'amount of such agent being preferably such that, the maximum bleaching effect will be producedwithout however effecting a substantial or serious decrease in the strength or in the yield of the pulp. The partially bleached pulp is then subjected to the action of a reducing bleaching agent containing anSO radical, and finally, after washing the pulp it is subjected'to a further treatment with a relatively small amount. of a chlorine agent,

' preferably ordinary bleaching powder. The improved method is thus carried out in three stages which, when ap lied to the bleaching of sulphate or kra pulp, are conducted substantially as follows :7

4 First stage.-The kraft pulp is bleached v with the requisite amountof bleaching pow:

mnrnon or nrm'onmo PAPER PULP.

Application filed December 24, 1925. Serial 1%. 17,622.

Thus, if the stock has been drastically pulped' a relatively smaller amount of bleach will be required than for a lowcooked or hard pulp. For most commercial purposes an amount of bleach equivalent to from 15 to 30 per cent of bleaching powder containing 35 per cent of available chlorine will be employed, such percentage being based upon the weight of the dry stock Usually, however for -a medium cooked stock, the amount of bleaching powder employed in the initial stage will varyfrom 18 to 23 per cent. The initial bleaching can be efl'ected with any of the difi'erent forms of bleacher now used in which the stock density varies from 4 to 16 per cent. The temperature employed may vary from ordinary room temperature to 104 degrees F, dependent upon the condition of thebleacher. That is to say, in a high density bleacher no'heat 'would be applied, but where the stock is to be bleached at low density, say 4% per cent,

in bleaching-powder and a somewhat better result can be produced if this initial bleaching is carried outfin two stages, i.- e., by adding about three-quarters of the total amount of bleaching agent for the preliminary treatment andfafter exhaustion of this 30 amount of bleaching powder, washing the pulp and then subjecting it to the remaining uarter of the total amount of bleaching pow er employed. This initial stage of the method bleaches the kraft pulp from its 5 characteristic brown color to a light yellow and since an ex'cessivegamount of bleaching agent is not employed its strength and yield are not materially impaired.

Secondttaga-After exhaustion. of the chlorine bleaching agent employed in the first stage, the pulp is treated with from 2 to 5 per cent of sodium bisulphite based on the dry weight of the stock and equivalent to from 40 to pounds per ton of dry pulp. 5 After subjecting the pulp to the action of the. sodium bisulphite for aboutone hour, it is well washed to efiect the removal of the decomposition products and any excess of bisulphite. The bisulphite can be added without washing out the bleach residues from the first stage of the treatment, but slightly better results are attained it' these products are first removed by washing. The bisulphite treatment removes the yellow color due to the chlorinated products that are formed in the first stage of the bleaching and which cannot readily be removed by direct treatment with bleaching powder or an equivalent chlorine bleaching agent. Such removal of the color of the chlorinated products is thoughtto be effected partially by the reduction of such products by the bisulphite and partially by the combination of the bisulphite therewith.

WVhile a suitable sulphite and preferably sodium bisulphite is employed, other reducing agents carrying an SO radical may be substituted.

T/lc'rd stage-The stock is now washed and is subjected to a final treatment of a shall amount of the chlorine bleaching agent. Preferably, bleaching powder is employed in the proportion of from 1 to 5 per cent by weight of the original dry stock, that is from 20 to 100 pounds of bleaching powder per ton of dry pulp. Usually about pounds will be found to produce satisfactory results when employing a usual form of bleacher. On exhaustion of the bleaching powder employed in this final stage, the stock is washed and handled in the usual way.

As stated, ordinary sulphate or kraft pulp cannot be satisfactorily bleached to produce a white color with a chlorine agent, but by treating the pulp in accordance with the present improved process, first with suflicient bleachlng powder to produce a maximum effect withoutseriously decreasing its yield or strength, and then treating it as described with a substantial *a-mount of sodium bisulphite, it is found that the bisulphite changes the yellow chlorinated coloring compounds produced in the first treatment, probably both by reduction and by direct addition thereto, so that the bulk of these yellow coloring compounds are removed by washing. It is then found that the pulp is in such condition that a very small amount of bleaching powder will bring the final product to a satisfactory white color.

As has been noted, satisfactory commercial results cannot be obtained by methods now generally in use in bleaching sulphate or kraft pulp to a white color. Further more, even when it is bleached to a very light manila, there is, with the usual methods, a loss in stren h of about 30 per cent upwards and a loss In yield ofapproximately 10 per cent. With the present improved method, it is found that a good white color can be produced, for example with a medium cooked kraft pu p, wit a loss in :t'orth without departure from the essentials 1 of the invention as defined in the claims.

I claim as my invention:

1. The method of bleaching sulphate or kraft pulp to a white color without substantial decrease in the strength or yield, which consists in efiecting a partial bleach with an oxidizing bleaching agent, then subjecting the partially bleached pulp to the action of a reducing bleaching agent to reduce and convert the contained coloring compounds, and then efi'ecting a final bleaching action by the use of a relatively small amount of an oxidizing bleaching agent, substantially as described.

2. The method of bleaching chemical wood pulp'which consists in subjecting the same to the action of a chlorine bleaching agent, then subjecting the same to the action of a reducing bleaching agent containing an SO radical and finally to the action of a relatively small amount of a chlorine bleaching agent, substantially as described.

3. The method of bleaching sulphate or kraft pulp which consists in subjecting the same to the action of a chlorine bleaching to produce the maximum efl'ect without substantial decrease in strength or yield, and completing the operation by subjecting the partially bleached pulp in successive stages to the action of bleaching agents containing an SO radical and chlorine respectively.

4. The method of bleaching chemical pulp which consists in treating the same initially with a hypo-chlorite in amount sufficient to produce a maximum effect without substantial reduction in strength or yield, and then treating the same, in successive stages, with sulphite and chlorine bleaching agents substantially as specified.

5. Themethod of producing a white sulphate or kraft pulp which consists in subjecting bleached sulphate pulp successively to the action of sodium bi-sulphite and bleaching powder substantially in the proportions of from ill) to 100 pounds of bi-sulphite and of from 20 to 100 pounds of bleaching powder per ton of dry pulp, substantially as described.

6. The methodof bleaching sulphate or kraft pulp which consists in treating the same in successive stages, first with from 15 to 25 per cent of bleaching powder, then, with from 2 to 5 per cent of sodium bi-sulphite and finally, with from 1 to 5 per cent of bleaching powder, substantially as described.

7 The method of bleaching chemical wood pulp which consists in treating the same in successive stages to an oxidizing chlorine bleaching agent and to a reducing bleaching agent containing an ing the pulp and radical, then washefiecting a' final bleach 5 with. a relatively small amount of a chlorine bleaching agent, substantially as described.

8. The method of pro ducing a white sulphate or kraft pulp which consists in treating the same in successive stages with bleaching powder and sodium bi-sulphite, then washing the same and completing the bleach with a relatively small amount of bleaching powder, substantially as described.

OTTO KRESS. 

